FIG. 8 shows an equivalent circuit of a prior art microwave oscillator manufactured as a monolithic microwave IC substrate (not shown). This oscillator is hereinafter referred to as an MMIC oscillator. This MMIC oscillator is generally indicated by numeral 1. The oscillation frequency of the oscillator 1 can be controlled by controlling its input voltage. The oscillator 1 comprises a signal amplifying portion 100 amplifying microwave signals and supplying a part of the output signal from the amplifying portion 100 as oscillation output to an external load 300, and a reactance control portion 200 which controls the output frequency of the amplifying portion 100 by varying the reactance component of the reactance control portion 200.
In the signal amplifying portion 100, a feedback amplifier element 10 provides feedback amplification of microwave signals. In this design, a GaAs MESFET is used as the amplifier element 10. This FET is hereinafter referred to as the feedback amplifier FET. A load-coupling portion 110 capacitively couples the feedback amplifier FET 10 to the external load 300, and comprises a source bias portion 111 applying a given bias potential to the source 10a of the FET 10 and a signal output portion 112 from which the source output is supplied to the external load. The source bias portion 111 is composed of a bias resistor R4 and a transmission line T5 which are connected in series between the source 10a and ground. The signal output portion 112 comprises a transmission line T6, a load-coupling capacitor C4, and another transmission line T7 which are connected in this order in the line going from the source terminal 10a to the external load. A capacitor C3 grounds the drain 10b of the feedback amplifier FET 10 at microwave frequencies and forms a grounding portion 120. A constant voltage source 11 applies a DC voltage V.sub.dd to the drain 10b.
The reactance control portion 200 includes a diode 210, shown as its equivalent circuit elements, inserted between the gate 10c of the feedback amplifier FET 10 and ground biased in the reverse direction such that its capacitive component C2 can be varied. The diode 210 has an internal resistance R1. Transmission lines T2 and T3 are connected in series between the gate 10c and the anode of the diode 210. A capacitor C1 is inserted between the cathode of the diode 210 and ground. The form a feedback path P.sub.FB for the source output between ground and the gate 10c of the feedback amplifier FET 10. A voltage source 21 produces a DC voltage V.sub.cont to control the capacitive component C2 of the diode 210 and the oscillation frequency. The voltage source 21 is connected to the cathode of the diode 210 through a transmission line T1. Another transmission line T4 is connected between ground and the junction of the transmission lines T2 and T3 to match the signal waveforms in the feedback path P.sub.FB. Resistors R2 and R5 prevent microwave signals from leaking out of the feedback path P.sub.FB.
The aforementioned external load 300 consists of a one-stage high-output amplifier 310 amplifying the oscillation output and an antenna 320 radiating the output from this amplifier 310 in the form of electromagnetic waves. The amplifier 310 comprises a signal amplifying FET 301. A transmission circuit 311 on the input side introduces the oscillation output to the gate of the FET 301. Transmission lines T8-T11 on the input side provide impedance matching. A capacitor C5 block DC components. A transmission line T12, capacitors C6 and C7, and a resistor R6 together form a gate bias circuit for the amplifying FET 301. A transmission circuit 312 on the output side guides the output from the drain of the FET 301 into the antenna 320. Transmission lines T13-T16 on the output side provide impedance matching. A capacitor C8 blocks DC components. A transmission line T17, capacitors C9 and C10, and a resistor R7 together form a drain bias circuit. All of these devices and transmission lines are monolithically manufactured as a monolithic microwave IC (hereinafter, referred to as an MMIC).
In the operation of this MMIC oscillator 1, when the voltage V.sub.dd (normally about 3 to 10 v) from the constant voltage source 11 is applied to the feedback amplifier FET 10, the input to the gate is amplified by the FET 10 and fed to the source 10a. The source output is applied to the gate. In this way, the signal is fed back and amplified. At this time, the microwave signal is fed back through the feedback path P.sub.FB and amplified. At the same time, a portion of the fed back and amplified output is supplied as the oscillation output to the external load 300 via a signal output path P.sub.OUT. In the external load 300, the oscillation output is amplified by the one-stage high-output amplifier 310. The output of this amplifier 310 is radiated as electromagnetic waves by the antenna 320.
When the oscillator oscillates in this way, if the voltage of the variable voltage source 21 of the reactance control portion 200 is varied, then the capacitive component C2 of the diode 210 varies, i.e., the reactance of the reactance control portion 200 changes. As a result, the frequency of the oscillation output is varied. In this manner, the frequency of this MMIC oscillator 1 is varied by altering the DC control voltage V.sub.cont.
Japanese Published Patent Application 60-224310 discloses an MMIC oscillator which controls the oscillation frequency by applying a control voltage to a varactor diode.
This prior art MMIC oscillator uses a metal-insulator-metal capacitor (hereinafter, referred to as MIM capacitor) as the load-coupling capacitor C4 of the load-coupling portion 110, it being noted that the MIM capacitor can form a large capacitive element. Therefore, the capacitance of the load-coupling capacitor C4 varies greatly among commercial product.
Specifically, it is generally difficult to obtain the previously calculated capacitance value from a capacitor such as an MIM capacitor. In the MIM capacitor, the interlayer insulating film is formed on the lower metallization layer, and the upper metallization layer is deposited on the insulating film. Since the capacitance value is in inverse proportion to the thickness of the interlayer insulating film, if the film thickness varies during the manufacturing process because of temperature variations occurring during the film formation or because of variations of the composition of the gas in the raw material gas supply source, then the capacitance also varies. As an example, when the load-coupling capacitance is too large an, oscillation output cannot be obtained in a voltage region of the variable voltage source 21 (oscillation halting region S shown in FIG. 9) or the oscillation frequency F.sub.OSC tends to change abruptly in a region of the control voltage (frequency jumping phenomenon J shown in FIG. 9). Conversely, when the load-coupling capacitance becomes too small, a decrease in the oscillation output often takes place.
To overcome these problems, a reference MMIC oscillator and several preliminary MMIC oscillators have been heretofore designed. In particular, the reference oscillator includes the MIM capacitor C4 acting as the load-coupling capacitor, and the capacitance of this capacitor C4 is set to a desired value. The values of the capacitors C4 of the preliminary oscillators ar set to different values deviating from the desired value within a range of about + 5%. After these oscillators are actually manufactured, the oscillation characteristics or the like are measured. Then, the MMI oscillator which exhibits the best characteristics is selected and mass-produced. In this method, the designing step prior to the mass production needs much labor. Also, this method is uneconomical to perform.
Published Patent Application 2-183606 discloses a feedback type MIC (Microwave Integrated Circuit) oscillator having a first transmission line forming a feedback amplifier circuit and a second transmission line for the output. A multi-land capacitor is disposed between the first and second transmission lines to facilitate optimizing the impedance at the junction of the two transmission lines. It may be thought that this multi-land capacitor disclosed in the aforementioned Japanese patent specification can be used a the load-coupling capacitor described above.
However, no specific structure of the multi-land capacitor is disclosed in the patent specification, and it is difficult to use the multi-land capacitor as the above-described load-coupling capacitor. Also, it is considered that fabricating the capacitor in the form of multiple lands is realized by preparing several capacitors and then the appropriate one out of the prepared capacitors. That is, this method involves preparing several capacitors having large capacitances which are necessary for the proper function. This is disadvantageous for integration and efficient utilization of the devices and does not meet the present demands for lower cost and larger-scale integration.